Transponder and reader system

ABSTRACT

A reader for identifying passive transponders includes an antenna for receiving a first signal (Tx) from the reader and for transmitting a second signal (Rx). The reader includes: a time base generating a sinusoidal signal, a control circuit, a modulator for transmitting to the control circuit data to modulate the sinusoidal signal so as to generate a signal carrying commands which, when it is received by the antenna of the transponder, forms the first signal (Tx), a demodulator, a decoder, a communication interface and an antenna. The demodulator, which is connected to the antenna to receive the second signal (Rx), includes two multipliers, one being of the inverter type, and an adder designed to combine the data derived from the multipliers. A system including first and second devices that communicate by way of a modulated transmitted signal is also described.

This application is a division of application Ser. No. 10/333,300, filedon Jun. 30, 2003 is now a U.S. Pat. No. 7,014,111. Application Ser. No.10/333,300 is the national phase of PCT International Application No.PCT/CH01/00445 filed on Jul. 17, 2001 under 35 U.S.C. § 371, whichclaims priority of French Application No. 00/09625 filed Jul. 21, 2000.The entire contents of each of the above-identified applications arehereby incorporated by reference.

The present invention relates to a reader for identifying passivetransponders of the type comprising an antenna arranged for receiving afirst signal Tx from the reader and for transmitting a second signal Rxto the reader. It concerns more particularly readers allowing receptionof low intensity signals.

Units comprising a reader and at least one transponder, and which allowcontactless identification, are well known to those skilled in the art.They are based on the principle of electromagnetic coupling. One of themis, for example, disclosed by EM Microelectronic—Marin SA (Switzerland)under the title “CID Demokit Application note on transceiver unit”.

The reader of this unit comprises an antenna and a control circuit whichtransmit a signal Lx formed of an amplitude modulated low frequencycarrier, typically comprised between 9 and 150 kHz. They receive, inreturn, the signal Rx.

More precisely, the reader is also provided with a time base forgenerating the carrier signal, the carrier being modulated by thecontrol signal, as well as with a demodulator for processing the pickedup signals and with a microcontroller for decoding them. In the reader,a tuning capacitor is connected to the antenna to form a seriesresonating circuit, tuned to the frequency source.

The transponder receives from the reader the signal Tx, whichcorresponds to the damped signal Lx and which provides it, at the sametime, with energy, commands and the clock. When the reader has finishedaddressing its commands, it continues to transmit the carrier signal,but without modulation. Consequently, the transponder is constantlysupplied with energy and driven by the clock.

The transponder addresses signal Rx to the reader, by periodicallyshort-circuiting its antenna in accordance with coded modulation thatcorresponds to the response to be given.

Rx is received superposed onto the carrier by the reader antenna. Thiscombined signal is processed and decoded by the demodulator.

When the reader is separated from the transponder by a metal screen, theelectromagnetic coupling no longer occurs directly between the twoantennae of the reader and the transponder, but indirectly, through themetal screen in which induced currents (eddy currents) flow.

The coupling between the reader antenna and the transponder antenna thenbecomes very weak. Experience has shown that, in order to ensure aconnection in such circumstances, the energy and modulation must betransmitted in a particularly efficient manner, and the demodulator mustbe very sensitive, capable of differentiating modulations in signals Rxof the order of 70 dB with respect to the carrier.

An object of the present invention is to propose a reader allowing suchsmall modulations to be differentiated. Thus, the demodulator, connectedto the antenna to receive the second signal that it picks up from thetransponder arranged in the vicinity includes two signal multipliers,one being of the inverter type, and an adder for combining the data fromthese multipliers.

In an advantageous manner, the demodulator includes:

-   -   a first channel formed by the first multiplier,    -   a second channel arranged in parallel with the first channel and        including the second multiplier, as well as a low pass filter        and a sampling and hold circuit,    -   an adder, which receives the signals from the two channels and        processes them,    -   an amplifier-filter, and    -   a comparator.

The demodulator is arranged such that the signals derived from the adderare processed by the amplifier-filter then by the comparator. They arethen addressed to the interface to deliver, at the output thereof, asignal carrying information from the transponder.

Particularly when the reader is separated from the transponder by ametal screen, the inductance of the reader antenna has to be as high aspossible (of the order of several mH) with a high magnetic fieldconcentration. This can be obtained by coiling the reader antenna on acore of soft magnetic material, for example ferrite.

Reading must also not be dependent upon problems that could be generatedby variations in the features of the reader antenna, for example itstime constant. Therefore, the reader is arranged such that the beginningof the pause occurs by interrupting the current, and thus the magneticfield, during its zero crossing. The voltage across the terminals of theantenna or the tuning capacitor is then at a maximum and kept for thebeginning of the next transmission.

The reader commands require all or part of the transponders present inthe electromagnetic field to send an identification return signal Rx.They are in binary form, with a series of bits that differ from eachother in length. A pause, as short as possible, is inserted between thebits.

More precisely, the reader transmits signal Lx, of constant amplitude,with a pause time T₀ or T₁, depending on whether it wishes to send a 0or a 1, the ratio between T₀ and T₁ being substantially equal to ½. Thepause times are reduced to a minimum, so that the energy provided ismaximum and the measuring time as short as possible. The transponderantenna receives signal Tx, which is a damped image of signal Lx, thesinusoid decreasing, and then increasing progressively respectively whentransmission of Lx stops and begins. This pause time must, therefore, besufficient for signal Tx across the transponder antenna terminals to bedamped to a voltage lower than the threshold voltage, in order to allowthe demodulation circuit to detect it.

When a metal screen is capable of being located between the reader andtransponder antennae, it is preferable to work at the lowest possiblecarrier frequency. This inevitably results in an increase in the pauseduration.

It is also an object of the present invention to ensure a reliableconnection between a reader and a transponder, in the shortest possibletransmission time and to be able to work even with a metal screenarranged between the reader and the transponder. It allows, moreparticularly, the pause duration to be reduced to a minimum with respectto that of a command bit and to make the features of the transponder andreader antennae independent. The reader is thus arranged such that thefirst signal Tx, generated by the control circuit, is formed of asuccession of bits each of them ending in a pause whose duration is lessthan or equal to six periods of the sinusoidal signal.

Other advantages and features of the invention will appear from thefollowing description, made with reference to the annexed drawing, inwhich:

FIG. 1 shows schematically a transponder and a reader for identifyingthe transponder,

FIG. 2 shows a part of the transponder of FIG. 1, and more particularlyits analogue circuit,

FIG. 3 illustrates schematically at a and b, diagrams of the low andhigh level extractors,

FIG. 4 shows respectively, on lines a to d, the voltage curves measuredat the terminals of the reader antenna, the transponder antenna and thehigh and low clock signals,

FIG. 5 shows schematically a reader according to the invention, a partof which is illustrated in more detail in FIG. 6.

The unit, shown in FIG. 1, includes a reader 10 and a transponder 12.Reader 10 includes, more precisely, an antenna 14 transmitting anelectromagnetic signal Lx, an electronic control circuit 16,advantageously a microcontroller, as well as a computer 18 connected toa keyboard 20 and to a screen 22.

Transponder 12 is formed of an antenna 24, an analogue circuit 25, alogic control circuit 26 and a memory 27. Analogue circuit 25, whichwill be described in more detail hereinafter, is connected to theantenna, to receive the signal picked up by the latter. This signalcarries out three functions, namely it:

-   -   supplies the transponder with electric energy,    -   provides a clock signal, and    -   transmits commands.

After processing, the signal is transmitted from analogue circuit 25 tologic circuit 26, by three connections 28 a, 28 b and 28 c, respectivelyensuring the transmission of energy, the clock and the serial inputfunction. Logic circuit 26 addresses data to analogue circuit 25, viaconnection 29 of the “serial output” type. It introduces and will searchfor data in memory 27, respectively via connections 30 a and 30 b.

The commands are processed by logic circuit 26 from the data receivedand that contained in memory 27. A response is addressed, by logiccircuit 26, to analogue circuit 25 so that it sends a return signal toreader 10.

As FIG. 2 shows, analogue circuit 25 includes a capacitor 32, an AC-DCconverter 34, a low level clock extractor 35 and a high level clockextractor 36, a demodulator 38 and a modulator 40.

Capacitor 32 forms, with antenna 24, a resonating circuit whose naturalfrequency is adjusted to the frequency of signal Lx transmitted byreader 10, and to whose terminals the inputs of converter 34, extractors35 and 36, and the outputs of modulator 40 are connected, viaconnections 33 a and 33 b, such that the signal received by thetransponder is applied to each of them and the signal transmitted by thetransponder is applied to the antenna. Demodulator 38 receives, fromantenna 24, its energy through converter 34, and the clock and commandsthrough extractors 35 and 36.

This Figure again shows supply connection 28 a, clock connection 28 band serial input connection 28 c, forming the outputs of analoguecircuit 25. Serial input connection 29, which allows data to beaddressed from logic circuit 26 to analogue circuit 25, and moreparticularly to its modulator 40, will also be noted.

Converter 34 is, in a manner well known to those skilled in the art,formed of a full-wave rectifier with a large energy capacitor, poweringa voltage stabilizer. It powers all of the parts of transponder 12.

The low level type extractor is made by means of two simple inverters 42and 43 in series, as shown in FIG. 3 a. Inverter 42 is formed of a PMOStransistor 42 a and a NMOS transistor 42 b. It is powered by astabilized voltage VDD provided by converter 34. Antenna 24 appliessignal Tx to its input 42 c. The dimensions of transistors 42 a and 42 bare calculated such that the threshold voltage Ub at which the circuitswitches, is close to 1 V.

An extractor of this type generates a clock signal each time that Tx isgreater than 1 V, this signal being interrupted when Tx descends belowthis level. Consequently, the clock signal is interrupted.

In normal operating conditions, the peak voltage Tx_(max) is generallyof the order of ten volts. With an antenna quality factor comprisedbetween 15 and 30, approximately 3 to 6 cycles are required after thepause for voltage Tx to be permanently below 1 V.

It would of course be possible to dimension the transistors such thatthreshold voltage Ub is higher. In this case, however, the transpondercan no longer react normally when the received signal is close to Ub.This consequently reduces its sensitivity.

FIG. 3 b shows in more detail the high level type extractor 36. It isformed of an input transistor 44, of the PMOS type and a peak valuerectifier 46, both connected to the antenna by line 33 a, of two currentsources 48 and 50 and two inverters 52 and 53, one 52 being polarized,and the other 53 being simple.

More precisely, peak value rectifier 46 is formed of a diode 46 a and acapacitor 46 b. Its input is connected to antenna 24 by connection 33 aand its output 46 c to inverter 52 to apply a voltage VData thereto,equal to peak voltage Tx_(max) of the signal received by the antennathrough connection 33 a.

Inverter 52 includes an input 52 a and an output 52 b, and two PMOS 52 cand NMOS 52 d transistors. Input 52 a is connected to antenna 24 throughtransistor 44 which offsets the voltage of the antenna downwards by avalue equal to its threshold value. Detection threshold Uh of the highlevel extractor is offset downwards with respect to VData by a valueequal to the difference in the threshold voltages of PMOS transistors 44and 52 c. The latter are dimensioned such that the threshold voltage oftransistor 52 c is several hundred mV higher than the threshold voltageof transistor 44. Consequently, the clock signal is interrupted as soonas the voltage of signal Tx received by the antenna drops by a valueequal to the difference between the two threshold voltages, whatever thevalue of peak voltage Tx_(max).

In order to be able to understand properly the operation of the clockextractors, FIG. 4 shows schematically respectively on lines a, b, c andd, signal Lx transmitted by reader antenna 14, signal Tx received bytransponder 12 and the high and low level clock signals CLKh and CLKb.

On line a, it will be noted that the reader antenna transmits asinusoidal signal, which is periodically interrupted, when the voltageis maximum.

When the voltage of signal Lx becomes constant, signal Tx at theterminals of antenna 24 of transponder 12 decreases, as can be seen online b, more or less quickly, the speed being lower the higher thequality factor. Peak voltage Tx_(max) is higher, the higher the receivedsignal. However, when the voltage exceeds a limit value, the signal issaturated.

At the start of a signal Lx transmitted by the reader, the high and lowlevel clock signal extractors 36 and 35 both respond very quickly, ascan be seen on lines c and d. However, in poor reception conditions,high clock signal CLKh may only appear after several periods of signalLx transmitted by the reader. When reader 10 interrupts transmission ofthe sinusoidal signal, it will be noted that signal Tx received byantenna 24 is damped slowly. This is due to the fact that the qualityfactor of the oscillating circuit, that if forms with capacitor 32, ishigh.

Because of this slow damping, several periods are needed before lowlevel extractor 35 reacts, whereas signal CLKh derived from high levelextractor 36 is interrupted in synchronism.

It thus appears clearly that if the signal received is regular andintense, one need only have a high level extractor in the transponder inorder to be able to considerably reduce the pause time. However, whenthe received signal is not saturated, it is then desirable to have highand low level extractors, which then allows a pause of short duration tobe guaranteed. It is thus possible to transmit the maximum amount ofenergy and a significant number of data even when the carrier frequencyis low.

FIGS. 5 and 6 show, in more detail, the structure of electronic controlcircuit 16 fitted to reader 10.

Electronic control circuit 16 is formed of a time base 54, a modulator56, a demodulator 58, a decoder 60, a communication interface 62 and acontrol circuit 64.

Time base 54 is connected to control circuit 64, by a connection 54 a,through which it supplies a sinusoidal signal of constant frequency,advantageously comprised between 9 and 150 kHz, which acts as thecarrier. Control circuit 64 receives from modulator 56, via a connection56 a, data which allow it to modulate the carrier signal to address datato a transponder arranged in proximity to the reader, via antenna 14which is connected to control circuit 64 by means of a connection 64 a.

Antenna 14 is connected to demodulator 58 by a connection 14 a. Thus,when the transponder replies to the reader data, the signal that itaddresses, picked up by antenna 14, is received by demodulator 58,through connection 14 a. Demodulator 58 processes this signal and thedata that it contains is addressed to decoder 60 through a connection 58a. Decoder 60 interprets this data on the basis of stored data andtransmits it to interface 62 through a connection 60 a. Interface 62 isconnected to the exterior, by a connection 62 a, formed for example ofan RS line 232, to ensure the transmission of commands and data toman-machine interfaces. It is also connected to modulator 56, by aconnection 62 b.

Thus, when an operator wishes to identify an object provided with atransponder and arranged in the field of reader 10, he gives an order bymeans of keyboard 20. This order is managed by computer 18 and sent toelectronic control circuit 16 via connection 62 a. Interface 62addresses this order to modulator 56. The latter co-operates withcontrol circuit 64 to modulate the carrier signal derived from time base54.

As has already been stressed hereinbefore, it is difficult to read thesignals received by antenna 14, since they are of a very low level.Demodulator 58, shown in detail in FIG. 6, allows efficient reading tobe ensured. It includes first and second channels 66 and 68, arranged inparallel, an adder 70 connected to the outputs of channels 66 and 68, anamplifier-filter 72 and a comparator 74 arranged in series at the outputof adder 70.

Channel 66 is formed of a multiplier 76. Channel 68 includes an invertertype multiplier 78, a low-pass filter 80 and a sampling circuit 82.

The two channels 66 and 68 are together connected to the antenna byconnection 14 a. They therefore both receive the signal UR(t)originating from antenna 14. This modulated signal includes twocomponents, one corresponding to the transmitted signal and the other tothe picked up signal, originating from the transponder. Decoder 58 hasthe task of extracting the signal X(t) that corresponds to the componentoriginating from the transponder.

In a first operation, the signal is multiplied by itself by multipliers76 and 78, the latter further inverting the resulting signal. In otherwords, signal US(t) derived from multiplier 76 is equal to the square ofUR(t), whereas the signal derived from multiplier 78 is equal, but withthe reverse sign.

Signal-US(t), derived from multiplier 78, is then processed, in aconventional manner, by means of low-pass filter 80, then by samplingcircuit 82.

In the device described, reader 16 is the master as regards thetransponder. In other words, the reader can find out at any moment whena transponder is likely to respond to an interrogation. Just before theresponse signal begins, sampling circuit 82 stores the mean value ofsignal US-(t-Δt) provided by filter 80. It is this stored signal that isadded to signal US(t). After amplification and filtering byamplifier-filter 72, then comparison by comparator 74, the result ofthis addition allows X(t), which includes all data derived from thetransponder, to be extracted, whereas the signal originating from thecarrier has been removed.

It is quite clear that the transponder and the reader as they have beendescribed can be subject to numerous variants, without thereby departingfrom the scope of the invention.

Thus, although the carrier frequency is relatively low, and owing to thefeatures of the reader according to the invention, used withtransponders like that described, it is possible to read passivetransponders in particularly unfavorable conditions, even through ametal screen, safely and quickly.

1. A reader for identifying passive transponders of the type includingan antenna for receiving a first signal from the reader and fortransmitting a second signal, comprising: a modulated carrier signalgenerator constructed and arranged to produce as an output a modulatedcarrier signal that represents data to be transmitted; an antenna thatreceives as an input the modulated carrier signal from the modulatedcarrier signal generator and transmits the modulated carrier signal asthe first signal; a demodulator connected to receive as an input amodulated said second signal transmitted to the antenna; and a decoderconnected to receive an output of the demodulator; wherein the modulatedcarrier signal generator is constructed and arranged to generate themodulated carrier signal so as to include pauses in the signal duringwhich the modulated carrier signal does not oscillate, the modulatedcarrier signal generator maintaining the modulated carrier signal at themaximum voltage during the pauses, wherein, the modulated carrier signalgenerator comprises: a time base that generates a sinusoidal signal; amodulator that transmits data; and a control circuit arranged to receivethe sinusoidal signal from the time base and the data from themodulator, the control circuit producing as an output the modulatedcarrier signal based on the sinusoidal signal and the data, and whereinthe modulated carrier signal generator is constructed and arranged sothat the modulated carrier signal represents a succession of bits withsaid pause inserted between adjacent said bits.
 2. The reader of claim1, wherein the demodulator comprises: first and second multipliers, eachof the first and second multipliers receiving the modulated secondsignal, the first multiplier being non-inverting, the second multiplierbeing inverting; and an adder arranged to receive an output of the firstmultiplier and a filtered, sampled output of the second multiplier.
 3. Asystem including first and second devices constructed to communicate byway of a modulated transmitted signal, wherein the first devicecomprises: a modulated carrier signal generator constructed and arrangedto produce as an output a modulated carrier signal that represents datato be transmitted; and a first antenna that receives as an input themodulated carrier signal from the modulated carrier signal generator andtransmits the modulated carrier signal; wherein the modulated carriersignal generator is constructed and arranged to generate the modulatedcarrier signal so as to include pauses in the signal during which themodulated carrier signal does not oscillate; wherein the second devicecomprises: a second antenna constructed to respond to the transmittedmodulated carrier signal to generate a received modulated carriersignal; a capacitor connected to the antenna to form a resonatingcircuit; a converter constructed and arranged to convert the receivedmodulated carrier signal into a rectified power signal; a clockextracting circuit comprising: a high level clock extractor; and a lowlevel clock extractor; wherein the high level clock extractor isconstructed and arranged to identify a beginning of each said pause inthe transmitted modulated carrier signal, the low level clock extractorbeing constructed and arranged to identify an end of each said pause inthe transmitted modulated carrier signal, wherein the modulated carriersignal as output by the modulated carrier signal generator oscillatesbetween maximum and minimum voltages, the modulated carrier signalgenerator maintaining the modulated carrier signal at the maximumvoltage during the pauses, wherein the high level clock extractoridentifies the beginning of each said pause in the transmitted modulatedcarrier signal by identifying when a peak of the received modulatedcarrier signal no longer exceeds an upper threshold value which istracked with the carrier signal level, and wherein the low level clockextractor identifies the end of each said pause in the transmittedmodulated carrier signal by identifying when the received modulatedcarrier signal exceeds a lower threshold value.
 4. The system of claim3, wherein the high level extractor comprises: a PMOS input transistoroperatively connected to the second antenna; a polarized invertercomprising a PMOS transistor and an NMOS transistor, provided with aninput and an output, and controlled by the input transistor; a peakvalue rectifier, comprising a diode and a capacitor; two current sourcesrespectively powering the input transistor and the polarized inverter;and a simple inverter; wherein the peak value rectifier includes aninput connected to the second antenna and an output connected to thepolarized inverter to apply thereto a supply voltage (VData) equal to apeak voltage (Tx_(max)) of the received modulated carrier signal and inwhich the input of the polarized inverter is connected to the secondantenna through the input transistor, which offsets the receivedmodulated carrier signal downwards, the high level extractor beingarranged such that its detection threshold (Uh) is defined by the supplyvoltage (VData) offset downwards by a value equal to a voltagedifference defined by threshold voltages of the two PMOS transistors,such that an output of the high level clock extractor is interrupted assoon as the peak voltage (Tx_(max)) of the received modulated carriersignal drops below the detection threshold (Uh).
 5. A reader foridentifying passive transponders of the type including an antenna forreceiving a first signal from the reader and for transmitting a secondsignal, comprising: a modulated carrier signal generator constructed andarranged to produce as an output a modulated carrier signal thatrepresents data to be transmitted; an antenna that receives as an inputthe modulated carrier signal from the modulated carrier signal generatorand transmits the modulated carrier signal as the first signal; ademodulator connected to receive as an input a modulated said secondsignal transmitted to the antenna; and a decoder connected to receive anoutput of the demodulator; wherein the modulated carrier signaloscillates between maximum and minimum voltages; wherein the modulatedcarrier signal generator is constructed and arranged to generate themodulated carrier signal so as to include pauses in the signal duringwhich the modulated carrier signal does not oscillate, the modulatedcarrier signal generator maintaining the modulated carrier signal at themaximum voltage during the pauses, wherein the demodulator comprises:first and second multipliers, each of the first and second multipliersreceiving the modulated second signal, the first multiplier beingnon-inverting, the second multiplier being inverting; and an adderarranged to receive an output of the first multiplier and a filtered,sampled output of the second multiplier.